As someone from the UK I am not at all surprised, there is no upside to him doing so. There is nothing he can say that will help, the committee will absolutely rip into him as their primary audience is the press and people of the UK, they don't really care about his answers to their questions. The problem is that the story isn't about FB making mistakes, or being hacked, or doing something they said they wouldn't. It's that people are working out what it is they've agreed to to use the service, and there is no spinning that.